The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale in liquid and gaseous products, thereby producing "retorted oil shale".
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen containing retort inlet mixture downwardly into the retort as an oxygen supplying gaseous combustion zone feed to advance the combustion zone downwardly through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the retort inlet mixture downwardly into the retort, the combustion zone is advanced downwardly through the retort.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass in the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products including gaseous and liquid hydrocarbon products and to a residual soild carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas containing combustion gas generated in the combustion zone, gaseous products produced in the retorting zone, gas from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process is also withdrawn from the bottom of the retort. The products of retorting are referred to herein as liquid and gaseous products.
The residual carbonaceous material in the retorted oil shale can be used as fuel for advancing the combustion zone through the retorted oil shale. When the residual carbonaceous material is heated to its spontaneous ignition temperature it reacts with oxygen. The portion of the retort where the greater part of the oxygen in the retort inlet mixture that reacts with residual carbonaceous material in retorted oil shale is consumed is called the primary combustion zone. It is characterized by a temperature which is higher than in other parts of the retort. As the residual carbonaceous material becomes depleted in the combustion process, the oxygen penetrates farther into the oil shale retort where it combines with remaining unoxidized residual carbonaceous material, thereby causing the combustion zone to advance through the fragmented oil shale.
The rate of retorting of the oil shale to liquid and gaseous products is temperature dependent, with relatively slow retorting occurring at 600.degree. F., and relatively rapid retorting of the kerogen in oil shale occurring at 950.degree. F. and higher temperatures. As the retorting of a segment of the fragmented oil shale in the retorting zone progresses and less heat is extracted from the gases passing through the segment, the combustion gas heats the oil shale farther on the advancing side of the combustion zone to retorting temperatures, thus advancing the retorting zone on the advancing side of the combustion zone.
It can be desirable to limit the oxygen content of the combustion zone feed to about 15%. At oxygen concentrations higher than about 15%, high primary combustion zone temperatures resulting in fusion of the oil shale can occur if a high volumetric flow rate of combustion zone feed is provided. Thus to reduce the oxygen content of air, which is presently the most economical source of oxygen, the air can be diluted with a portion of off gas generated by retorting of oil shale. However, it has been found that when recycled off gas is used to dilute the air, the off gas from the retort can have a fuel value of only about 45 BTU/SCF (British thermal units per standard cubic foot), which can be insufficient to power a work engine.
It is desirable to provide a method for retorting an in situ oil shale retort such that the retort off gas generated during retorting has sufficient fuel value for combustion in a stack or for use in power generation in a work engine.
When off gas recycling is used to dilute air, a narrow combustion zone is generated. With off gas recycling, it is calculated that for a combustion zone having a maximum temperature of 1400.degree. F., the thickness of a primary combustion zone having a temperature of 1300.degree. F. on its leading edge and a temperature of 900.degree. F. on its trailing edge can be about 0.8 foot. With such a narrow primary combustion zone, oxygen from the primary combustion zone can oxidize hydrocarbon products produced in the retorting zone, thereby lowering the hydrocarbon yield from the retort.
The introduction of a gaseous retort inlet mixture into the retort on the trailing side of the combustion zone and the flowing of such gas therethrough generally reduces the temperature of the fragmented permeable mass of particles on the trailing side of the combustion zone. When the retort inlet mixture feed is introduced into the retort at atmospheric temperature, the fragmented permeable mass on the trailing side of the combustion zone can have its temperature reduced to a temperature below the retorting temperature of oil shale. This reduction in temperature terminates the retorting of oil shale in unfragmented formation adjacent to such fragmented permeable mass of particles, thereby reducing the recovery from the retort.
This can also reduce the temperature of residual carbonaceous material in oil shale on the trailing side of the primary combustion zone to a temperature below the spontaneous ignition temperature of such materials. The residual carbonaceous material cannot be oxidized to provide the energy required for the endothermic retorting of oil shale, thereby requiring oxidation of kerogen which otherwise could be retorted to yield hydrocarbon products.
Thus, it is desirable to provide a method for recovering liquid and gaseous products from an in situ oil shale retort which yields off gas of sufficient fuel value to operate a work engine and which gives high recovery of product.